In 3GPP LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is employed as a downlink communication method. In a radio communication system adopting 3GPP LTE, a base station transmits a synchronizing signal (synchronization channel: SCH) and a broadcast signal (broadcast channel: BCH) using prescribed communication resources. A terminal first synchronizes with a base station by capturing the SCH. Then, the terminal acquires parameters that are specific to that base station (for example, a frequency bandwidth) by reading BCH information (see, for example, Non-patent Literature 1, 2 and 3).
Also, after acquiring base station-specific parameters, a terminal to support an LTE system (hereinafter “LTE terminal”) sends a connection request to the base station, and, by this means, establishes communication with the base station. The base station transmits control information to the terminal with which communication has been established, by using a PDCCH (Physical Downlink Control CHannel) when necessary.
The terminal performs “blind detection” for a received PDCCH signal. That is, a PDCCH signal includes a CRC (Cyclic Redundancy Check) part, and, at a base station, this CRC part is masked by the terminal ID of the target terminal. Thus, until a terminal demasks the CRC part of a received PDCCH signal with the terminal's terminal ID, the terminal cannot decide whether or not the PDCCH signal is for that terminal. In this blind detection, if the result of demasking is that CRC calculation is OK, the PDCCH signal is decided to have been sent to the terminal.
Also, control information sent from a base station includes assignment control information including such as information about resources which a base station assigns to a terminal. A terminal needs to receive both downlink assignment control information and uplink assignment control information which have a plurality of formats. Although downlink assignment control information which a terminal should receive can be defined in a plurality of sizes depending on the transmission antenna control method and frequency assignment method at a base station, some of these downlink assignment control information formats (hereinafter simply referred to as “downlink assignment control information”) and uplink assignment control information formats (hereinafter simply referred to as “uplink assignment control information”) are transmitted using PDCCH signals having the same size. A PDCCH signal includes type information of assignment control information (for example, a 1 bit flag). Thus, even if the size of a PDCCH signal including downlink assignment control information and the size of a PDCCH signal including uplink assignment control information are the same, a terminal checks type information of assignment control information, and by this means can distinguish between downlink assignment control information and uplink assignment control information. The PDCCH format to transmit uplink assignment control information is PDCCH format 0, and the PDCCH format to transmit downlink assignment control information, transmitted in a PDCCH signal being the same size as uplink assignment control information, is PDCCH format 1A.
However, cases might occur where the information size of uplink assignment control information determined from the uplink bandwidth (that is, the number of bits required for transmission) and the information size of downlink assignment control information determined from the downlink bandwidth differ. To be more specific, if an uplink bandwidth is small, the information size of uplink assignment control information becomes small, and, if a downlink bandwidth is small, the information size of downlink assignment control information becomes small. If a difference of bandwidths results in a difference of information sizes like this, by adding zero information to the smaller assignment control information (that is, by performing zero-padding), the size of downlink assignment control information and the size of uplink assignment control information are made equal. By this means, whether the content is downlink assignment control information or uplink assignment control information, PDCCH signals have the same size.
The size adjustment of control information as mentioned above reduces the number of times of blind detection at a terminal on the reception side. However, when a downlink transmission bandwidth of a base station is wide, the base station transmits many PDCCH signals at once, so that the terminal cannot reduce enough the number of times of blind detection in its normal operation, and the increase of circuit scale of a terminal causes a problem.
Therefore, to further reduce the number of times of blind detection by a terminal, a terminal employs the method to limit a physical space where a terminal receives control information. Thus, each terminal is reported in advance the time and frequency space where control information for that terminal is likely to be included, and performs blind detection only in a terminal-specific space where control information for that terminal is likely to be included. This terminal-specific physical space is called “UE specific search space (UE SS).” This UE specific search space is associated with, for example, a terminal ID. Also, a time and frequency interleaving is employed to provide a nearly equal time diversity and frequency diversity effect in all UE specific search spaces.
Meanwhile, a PDCCH signal includes control information that is reported at once to a plurality of terminals (for example, scheduling information about downlink broadcast signals). To transmit this control information, a physical space that is common to all LTE terminals that should receive the target downlink broadcast signal, called “common search space (Common SS),” is prepared in a PDCCH signal. Even in this common search space, as in the UE specific search space, an information size adjustment is performed between the size of downlink assignment control information and the size of uplink assignment control information. Thus, even in common search space, it is possible to transmit uplink assignment control information to the terminal without increasing the number of times of blind detection by the terminal.
Thus, a terminal requires both control information included in a UE specific search space and control information included in a common search space, so that the terminal needs to perform blind detection for all uplink control information and downlink control information included in the UE specific search space and uplink control information and downlink control information included in the common search space.
FIG. 1 shows the transmissions of PDCCH signals by format 0 and format 1A. In FIG. 1, as mentioned above, in each UE specific search space and common search space, PDCCH signals are transmitted by format 0 and format 1A. In FIG. 1, the downlink bandwidth is 15 MHz and the uplink bandwidth is 20 MHz. The size of assignment control information depends on the bandwidth, so that, when comparing the information size (the size of format 1A) required for the downlink assignment decided from the downlink bandwidth and the information size (the size of format 0) required for the uplink assignment decided from the uplink bandwidth, the latter is larger. Thus, the pairs of a downlink band and an uplink band are used between the base station and the terminal as shown in FIG. 1, zero padding is performed to the downlink assignment control information to adjust the size of format 1A to the size of format 0.
Also, the standardization of 3GPP LTE-advanced has been started to realize faster communication than 3GPP LTE. A 3GPP LTE-advanced system (hereinafter referred to as “LTE-A system”) follows a 3GPP LTE system (hereinafter referred to as “LTE system”). In 3GPP LTE-advanced, to realize a downlink transmission speed equals or exceeds maximum 1 Gbps, a base station and a terminal which can communicate in wideband frequency of 40 MHz or more are expected to be introduced.
With an LTE-A system, to realize a communication by ultra fast transmission speed that is several times as fast as transmission speed in an LTE system, and backward compatibility for the LTE system at the same time, a band for the LTE-A system is divided into “component bands” that are LTE supporting bandwidths and that are equal to or lower than 20 MHz. “Component band” is a bandwidth for maximum 20 MHz here and is defined as the basic unit of a communication band. Furthermore, “component band” in a downlink (hereinafter referred to as “downlink component band”) is defined as a band separated by downlink frequency bandwidth information in a BCH broadcasted from a base station, or a band defined by the range of distribution when a downlink control channel (PDCCH) is arranged in a distributed manner. Also, “component band” in an uplink (hereinafter referred to as “uplink component band”) is defined as a band separated by uplink frequency bandwidth information in a BCH broadcasted from a base station, or the basic unit of a communication band of 20 MHz or less including a PUSCH (Physical Uplink Shared CHannel) near the center, and a PUCCH for an LTE on both ends. Also, in 3GPP LTE-Advanced, “component band” may be designated as “Component Carrier(s)” in English.
In an LTE-A system, the communication using the bandwidth that bundles a few of these component bands, so-called “Carrier aggregation” is supported. Generally, throughput requirements for an uplink and a downlink are different, so that in an LTE-A system, studies are underway to use the carrier aggregation in which the number of component bands set for an arbitrary terminal (hereinafter “LTE-A terminal”) associated with the LTE-A system, so-called “asymmetric carrier aggregation.” Furthermore, the case will also be supported where the numbers of component bands are asymmetric between an uplink and a downlink, and where all component bands have different frequency bandwidths.
FIG. 2 shows an asymmetric carrier aggregation applied to a dedicated terminal and its control sequence. FIG. 2 shows an example in which the bandwidths and the numbers of component bands of an uplink and a downlink of the base station are symmetric.
In FIG. 2, although as for terminal 1, a configuration is carried out to perform the carrier aggregation using two downlink component bands and one uplink component band of the left side, as for terminal 2, a configuration is carried out to use the uplink component band of the right side for the uplink communication, even though a configuration is carried out to use the same two downlink component bands as in terminal 1.
When focusing on terminal 1, between the LTE-A base station and the LTE-A terminal that form an LTE-A system, the transmission and reception of signals are performed according to the sequence diagram shown in FIG. 2A. As shown in FIG. 2A, (1) when starting communication with a base station, terminal 1 synchronizes with the left side downlink component band and reads from broadcast signals called SIB2 (System Information Block Type 2), the information of an uplink component band that forms a pair with the left side downlink component band. (2) By using this uplink component band, terminal 1, for example, sends a connection request to the base station, and by this means starts communication with the base station. (3) When determining that a plurality of downlink component bands need to be assigned to a terminal, the base station commands the terminal to add downlink component bands. However, in this case, the number of an uplink component band does not increase, and asymmetric carrier aggregation starts in terminal 1, that is a dedicated terminal.